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Video: Lawyer faces scrutiny

The fingerprints of the American lawyer arrested in Oregon in connection with the Madrid terror bombings were found on a bag containing detonators of the kind used in the March 11 attack, the Spanish government said Friday.

The plastic shopping bag was found inside a stolen van left near the train station from which three of the four bombed trains departed, an Interior Ministry official said.

U.S. law enforcement officials said that Brandon Mayfield, a lawyer and former Army officer who converted to Islam, was arrested as a material witness on Thursday in connection with the deadly train bombings in Spain. FBI agents also searched his home in the Portland suburb of Aloha.

Detonators were same type used in attackThe detonators in the bag were of the same kind used in the March 11 attacks, which killed 191 people, the government has said previously.

Global dragnetThe van was found in Alcala de Henares, about 20 miles northeast of Madrid. Inside it police also found an Arabic-language cassette tape with verses from the Quran, police have said.

It was the first known arrest in the United States with connections to the March 11 terrorist attacks in Madrid that killed 191 people and injured 2,000 others.

Mayfield, 37, has not been charged with any crime, according to law enforcement officials in Washington D.C., who spoke with numerous news organizations on the condition of anonymity.

A material witness warrant allows the government to hold people suspected of having direct knowledge about a crime or to allow time for further investigation into the witness.

Senior law enforcement officials, who spoke with NBC News on condition of anonymity, said the FBI matched the fingerprints to Mayfield through its fingerprint database, adding that his prints were in the system because he served in the U.S. military.

Lawyer was under surveillanceThe FBI put Mayfield under surveillance, but was forced to move in when it learned that word of the potential link to an American was beginning to leak to the Spanish news media, they said.

Beth Anne Steele, a spokeswoman for the FBI in Portland, confirmed two search warrants had been served Thursday in Washington County, which includes Aloha. She would not release further details.

Mayfield passed the Oregon bar in 2000 and largely kept a low profile in the Portland legal community until 2002, when he volunteered to represent Muslim terrorism suspect Jeffrey Battle in a child custody case.

Battle was among six Portland area residents who were sentenced last year on charges of conspiring to wage war against the United States by helping al-Qaida and the former Taliban rulers of Afghanistan.

Mayfield was not involved with Battle’s defense in that case. Law enforcement officials in Washington did not know of any contacts between Mayfield and the other Portland terrorism defendants.

Mayfield had converted to Islam in the late 1980s and regularly attended Friday prayers at a Beaverton mosque, said mosque administrator Shahriar Ahmed.

Wife proclaims husband's innocenceFriends and family said they were shocked to learn of the arrest.

Outside their home near Portland late Thursday, Mayfield’s wife, Mona, proclaimed her husband’s innocence and described him as “a good man, a good father, a good husband.” The couple have two sons, ages 10 and 15, and a 12-year-old daughter.

Portland attorney Tom Nelson, who described himself as a mentor, said he received a call Thursday afternoon from Mayfield asking for help.

“His wife was in tears because of the way the search was conducted. The FBI apparently hurt things in the house, left things in disarray,” Nelson told reporters outside Mayfield’s home. “He is a regular, run-of-the-mill guy.”

“Obviously, the government holds all the cards in these kinds of situations,” Nelson told ABC’s “Good Morning America” Friday. “It can release any kind of information it thinks it wants to release and the other side is prohibited to speaking on the merits so I can’t speak to the merits.”

“He’s in no position, to, say, do forensic tests of his own” on the alleged fingerprints, Nelson added. He said he was speaking as a friend and was not acting as Mayfield’s legal counsel.

Spanish authorities blame the attack on Islamic extremists, possibly linked to al-Qaida. Eighteen people have been charged to date, six with mass murder and the others with collaboration or with belonging to a terrorist organization.

NBC News' correspondent Pete Williams and the Associated Press contributed to this report.